Conventionally, for example, an electric power steering apparatus includes an electric motor for imparting a steering assist torque to a rotating operation of a steering wheel, and generates a proper steering assist torque through control of electricity supply to the electric motor. The steering assist torque is set such that the greater the steering torque input to the steering wheel by a driver and the lower the vehicle speed, the greater the steering assist torque. The electric power steering apparatus uses an onboard battery as a power source for performing such steering assist control.
However, at the time of stationary swing or the like, sole use of the onboard battery may fail to provide sufficient electric power. In view of such a problem, an apparatus proposed in Japanese Patent Application Laid-Open (kokai) No. 2007-91122 includes a secondary power supply apparatus for supplementing the onboard battery. The secondary power supply apparatus is connected in parallel with a power supply line extending from the onboard battery (hereinafter referred to as the “primary power supply apparatus”) to a motor drive circuit.
In this proposed electric power steering apparatus, electric power for driving the motor is withdrawn from the primary power supply apparatus. However, when sole use of the power supply of the primary power supply apparatus fails to provide sufficient electric power (e.g., at the time of stationary swing), electric power is withdrawn from the secondary power supply apparatus so as to make up for the shortfall. Further, when power consumption is small (e.g., at the time of straight traveling), electric power from the primary power supply apparatus is charged into the secondary power supply apparatus.